As an example of a display device, an organic EL (electroluminescence) display device is represented. An organic EL display device possesses a light-emitting element in each pixel, and a light-emitting element has a layer (hereinafter, referred to as an EL layer) between a pair of electrodes. A light-emitting element is driven by supplying current between a pair of electrodes. During operation of a light-emitting element, the organic compound is oxidized or reduced with the current flowing in the light-emitting element to be transformed into a state having an electric charge. Furthermore, an excited state is generated by recombination of these active species. Such active species readily react with other organic compounds or impurities such as water and oxygen, which enter the light-emitting element, due to their higher reactivity than that of a molecule in an electrically neutral state or a ground state. The reaction with the impurities adversely influences performance of a light-emitting element, resulting in reduction of efficiency and lifetime. The reduction in emission efficiency finally inhibits emission to be obtained, and a pixel from which emission cannot be obtained is recognized as a black spot (dark spot) during operation of a display device.
Since a light-emitting element of an organic EL display device is sealed with a pair of substrates, the impurities mainly enter from an edge portion of a display device. Influence of the impurities cannot be found immediately after manufacture, and the influence of the impurities may appear after a few days or a few months. In such a case, a display defect may occur a little while after shipment even in the case of a display device which is recognized as a quality item when manufactured.
With respect to such a problem, a technique is disclosed in Japanese patent application publications No. 2010-272270 and 2012-79658 for detecting the entrance of the impurities at an early stage. As another solution, a method is disclosed in which, although the entrance of the impurities cannot always be detected, the impurities entering a display device are trapped to suppress reduction of performance (see, Japanese patent application publications No. 2002-8852 and 2006-80094).